1. Field of the Invention
This invention relates to a hydrogen fuel hose. More particularly, it relates to a hydrogen fuel hose which can overcome various problems such as the embrittlement of its wall material by hydrogen, the effects of steam on a hose in a fuel-cell vehicle and the extraction of a substance which is a catalyst poison in a fuel cell, and a fuel hose which has a high impermeability to hydrogen gas and flexibility as well.
2. Description of the Related Art
The development of a fuel-cell vehicle as a next-generation vehicle is under way since the problems such as environmental pollution and oil shortage have been spotlighted. Hydrogen gas is a typical fuel for it. Methanol, methane, etc. can also be used if a reformer is available to generate hydrogen. Further, attention has also been paid to dimethyl ether as a clean fuel for a diesel engine. Dimethyl ether has a high conversion efficiency as a hydrogen supply source since it is easily liguefied under pressure.
Hydrogen gas requires careful handling because of its high permeability and combustibility. Hydrogen gas is still difficult to use independently as fuel, and is usually used with hot steam. Steam is a source of water for an electrolyte membrane for achieving an improved efficiency of power generation, or a source of hydrogen for a fuel cell of the type relying upon the reforming of a hydrocarbon, such as methane.
A metal tube, such as a stainless steel tube having a large wall thickness, is a typical hydrogen fuel hose to avoid problems including embrittlement by hydrogen. A metal tube is also considered effective for methanol, methane, etc. to avoid the problems of corrosion and fuel permeation. A metal tube having a thick wall of high rigidity, however, is not desirable as a hydrogen fuel hose for a fuel-cell vehicle. Such a tube is contrary to the requirements for a reduction of vehicle weight and fuel consumption, cannot absorb the vibration of a running vehicle satisfactorily, and restricts freedom in piping layout. A mere reduction in wall thickness of a metal tube is not desirable, either, since it becomes lower in strength and pressure resistance, and more liable to corrosion by exposure to hot steam, or embrittlement by hydrogen.
According to Japanese Patent Application Laid-Open No. 275981/1995, a straight metal pipe is coated with a resin layer by thermal shrinkage or extrusion, and then the pipe is corrugated. The corrugated pipe is useful for the absorption of vibration and the ease of installation. Japanese Patent Application Laid-Open No. 127101/1996 discloses a pipe used for conveying hot water for bathing, room heating, etc. It is a metal pipe having an outer wall surface coated with a powder of a modified polyolefin resin and a polybutene resin. The resin coating is effective for maintaining the strength of the wall having a reduced thickness, or protecting the outer wall surface from corrosion.
Any of the known metal pipes, however, has its inner wall surface exposed to hydrogen fuel and hot steam, though its outer wall surface is protected by a resin layer. The embrittlement of the metal by hydrogen or its corrosion is likely to occur easily and thereby lower the strength of the wall. The dissolution of metal ions, etc. also occurs, which may cause the deterioration of the catalyst in a fuel cell by pollution. Moreover, the exposed metal surface is likely to spoil the electrical insulation of a fuel cell and cause a leak of electricity therefrom.
The researches by the inventors of this invention reveal that a layer of a modified polyolefin resin or a polybutene resin even if formed on the inner surface of a metal pipe is unsatisfactory as a hydrogen gas barrier, though it is satisfactory in waterproofness. Hydrogen gas passes through the resin layer and embrittles the metal wall.
Moreover, the prior art does not provide any effective suggestion about the design standards for the preparation of corrugated hoses from metal pipes. For example, there is no teaching about the standard for the flexibility of a hose which makes it capable of absorbing vibration effectively or easy to install, or the maximum metal wall thickness that is allowable for its flexibility, or the minimum resin layer thickness required for the satisfactory strength of the hose.
It is an object of this invention to provide an effective measure to overcome embrittlement by hydrogen as a problem peculiar to a hydrogen fuel hose in a fuel-cell vehicle. It is another object of this invention to provide an effective measure to avoid the influence of steam and the extraction of a catalyst poison as other peculiar problems. It is still another object of this invention to provide an effective measure to improve the hydrogen impermeability, flexibility and strength of a hose.
According to a first aspect of this invention, there is provided a hydrogen fuel hose having a wall comprising an inner resin layer, a thin metal layer and an outer resin layer, wherein the inner resin layer includes a layer of low gas permeability formed from a resin having a helium gas permeability not exceeding 10xe2x88x929 cm3xc2x7cm/cm2xc2x7secxc2x7cmHg at a temperature of 80xc2x0 C.
The thin metal layer ensures the high hydrogen gas impermeability of the hose, as well as its flexibility. The inner resin layer ensures that no metal ion be dissolved from the metal layer, thereby preventing pollution deterioration of a fuel cell catalyst. The layer of low gas permeability in the inner resin layer ensures that the metal layer be kept away from a gaseous mixture of hydrogen and steam and thereby protected from corrosion. The inventors of this invention have found that it is possible to prevent any embrittlement of the metal layer by hydrogen if the layer of low gas permeability has a helium gas permeability within the range stated above. The outer resin layer improves the strength, durability, chipping resistance and electrical insulation of the hose.
According to a second aspect of this invention, the inner resin layer further includes a layer of low water permeability surrounded by the layer of low gas permeability and having a pure water permeability not exceeding 5 mgxc2x7mm/cm2/day. Although the layer of low gas permeability is usually an effective hydrogen gas barrier, its performance may be somewhat lowered by water, and if water reaches the metal layer, its corrosion may be promoted. These problems can, however, be overcome by the layer of low water permeability, particularly if its pure water permeability falls within the range stated above.
According to a third aspect of this invention, the layers of low gas and water permeability are of a resin not containing any halogen. If any halogen were extracted f rom any such resin layer, it could be a strong poison causing the deterioration of the catalyst in a fuel cell.
According to a fourth aspect of this invention, the hose is corrugated at least along a part of its length and has an axial spring constant not exceeding 25 N/mm. The inventors have experimentally ascertained that a corrugated hose having an axial spring constant not exceeding 25 N/mm can absorb vibration satisfactorily and is easy to install.
According to a fifth aspect of this invention, the metal layer has a thickness of 20 to 300 xcexcm. A metal layer having a thickness of less than 20 xcexcm may fail to make an effective hydrogen gas barrier. A metal layer having a thickness over 300 xcexcm may not produce any better result, but merely gives a hose having an undesirably heavy weight and an undesirably high spring constant.
According to a sixth aspect of this invention, the inner and outer resin layers have a total thickness B having the following relationship to the thickness A of the metal layer:
A:B=1:4 to 1:50
This is the range in which the resin layers can reinforce the metal layer most effectively. The resin layers may be unsatisfactory for reinforcing purposes if B is less than four times as large as A. If B is over 50 times as large, however, the resin layers may not produce any better result, but merely give a hose having an undesirably heavy weight.
According to a seventh aspect of this invention, the inner and/or outer resin layer is of a resin having a volume specific resistance of at least 1010 xcexa9xc2x7cm. The inner and outer resin layers ensure the electrical insulation of the hose so that there may not occur any leak of electricity from the fuel cell through the hose, particularly if at least one of the resin layers is of a resin having a volume specific resistance within the range stated above.
According to an eighth aspect of this invention, the hose is characterized by at least one of the following features:
(1) The resin forming the layer of low water permeability is polypropylene (PP), acid-modified PP (e.g. maleic anhydride-modified PP), polyphenylene sulfide (PPS), polyamide 6T (PA6T), polyamide 9T (PA9T), polyamide 612 (PA612), polyethylene (PE), acid-modified PE (e.g. maleic anhydride-modified PE), polymethylpentene, polyether sulfide (PES), polyether ether ketone (PEEK) or a copolymer of m-xylylenediamine and adipic acid (MXD6);
(2) The resin forming the layer of low gas permeability is an ethylene-vinyl alcohol copolymer (EVOH), polybutylene naphthalate (PBN), polyamide 6 (PA6), a polyamide 6-polyamide 66 copolymer (PA6-66), a PA6 or PA6-66 nanocomposite, PA6T, PA9T, PA612, polyamide 46 (PA46), PPS, PES or MXD6;
(3) The metal layer is of stainless steel, iron, an iron alloy, aluminum, an aluminum alloy, copper, a copper alloy nickel or a nickel alloy; and
(4) The resin forming the outer resin layer is PP, acid-modified PP, PPS, polyamide 12 (PA12), polyamide 11 (PA11), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), PA6T, PE, acid-modified PE, PA9T, PA612, PA912, PES, PEEK or MXD6.
The eighth aspect of this invention provides preferable examples of materials for the layer of low water permeability, the layer of low gas permeability, the metal layer and the outer resin layer.
The above and other features and advantages of this invention will become more apparent from the following description and the accompanying drawings.